This invention relates to an epoxy resin powder coating composition and a process for the preparation thereof.
Epoxy resin powder compositions have been generally prepared by the dry blending method, melt blending method or press-bonding blending method. In the dry blending method, an epoxy resin powder and a curing agent powder are mixed with each other as such. While the dry blending method is advantageous because the mixing can be easily carried out, this method has a problem because the epoxy resin and the curing agent tend to be separated from each other during transportation or use due to the difference in specific gravity and/or particle size therebetween.
The melt blending method includes the steps of mixing a melted epoxy resin with a melted curing agent, cooling and solidifying the resulting mixture, and pulverizing the solidified mixture. The melt blending method is free of such a problem of separation of the respective ingredients as seen in the dry blending method. However, another problem is encountered because crystal structures of the epoxy resin and/or the curing agent are destroyed during the melting stage. The presense of crystalline ingredients is highly desired because the composition can exhibits a low viscosity when melted during the curing stage so that it can smoothly penetrate into gaps to be filled therewith for bonding.
The press-bonding blending method is a modification of the above dry blending method and includes the steps of dry-blending epoxy resin powder with curing agent powder, pressing the resulting blend at a high pressure to adhere these ingredients to each other, and pulverizing the pressed blend. Though the resulting composition has a less tendency to cause separation of ingredients as compared with the dry blending method, it still involves such a problem because the adhesion between the ingredients is not sufficiently strong.